out of level can be a big problem *IF* it makes the stand off level or allows it to "Flex" or "Bend" in any way....
depending on the size of the tank and stand and how much the floor is "out of level" you may need to do different things....
one extreem is to re-tile the floor
possibly get a large section of plywood to set the stand on and then put shims under the plywood and level that so that the whole stand is on a level surface.
one time I used shims on a stand on carpet to good results...
you can get shims at any hardware store like home depot.
also I have a "Post Level" that helps.
it's a set of levels built on a plastic housing that you can hang on the top of the tank and go around the tank checking left-right and front-back at one time...
also folks use sheets of styrafoam .... between a tank and a stand.
but I perfer to start at the floor and get that solid first!